The present invention relates to ink jet printers and, more particularly, to a circuit and method of operation for use in such a printer which provides for optimum charging of drops and drop placement in dependence upon movement of a print receiving medium.
Ink jet printers, such as shown in U.S. Pat. No. 3,701,998, issued Oct. 31, 1972, to Mathis, and assigned to the assignee of the present invention, are known in which one or more jet drop streams of electrically conductive ink are produced by forcing the ink under pressure through an orifice or nozzle of a print head. The ink flows through the nozzle and emerges as a fluid filament which, in the absence of mechanical stimulation, would break up into drops of varying size and spacing.
It is known, however, that if mechanical stimulation is applied to either the ink, the nozzle, or the print head, as by a piezoelectric or other electromechanical transducer, the fluid filament may be caused to break up into drops of substantially uniform size and spacing. Further, if a charge electrode is placed in the vicinity of the fluid filament tip, from which the drops are formed, and a charge voltage applied to the electrode, an induced charge in the filament tip is carried away as each successive drop is formed.
By selective application of the charge voltage to the electrode, various ones of the drops produced by the jet drop stream may be charged to one or more levels, while others of the drops are left uncharged. The drops thereafter pass through an electrostatic field which deflects the charged drops in dependence upon the electrical charge which they carry. The drops are thus separated into at least two trajectories, with the uncharged drops passing unaffected through the field. The drops in one of the trajectories are deposited upon a moving print receiving medium at a print station, while the drops in the other trajectory or trajectories are caught by a drop catcher to prevent their deposit on the print receiving medium. If the drops are charged to more than two levels, they will be selectively deflected into more than two trajectories. This permits the drops from a single jet drop stream to be deposited at more than one position. A number of jet drop streams may be provided. In the printer using a single jet drop stream, the print head transport or the transport for the print receiving medium or both are arranged such that drops from the jet may be deposited at a large number of points across the medium. In either type of printer by selective charging and catching of the drops, a number of drops may be deposited upon the print receiving medium in a manner so as to form collectively a print image.
It will be appreciated that the quality of the print image will be dependent upon a number of different factors. If the timing of the application of charge signals to the charge electrode is not synchronized with the timing of drop formation, it will be appreciated that when a drop is formed while the charge electrode potential is being switched from one charge voltage to another, the drop will be improperly charged and, as a consequence, will not undergo the desired deflection, resulting in misplacement. Further, it will be appreciated that any fluctuations in the velocity of the print receiving medium past the print station will result in drops striking the medium at points other than desired.
Finally, it will be appreciated that as a drop is being formed from the fluid filament, the charge induced on the drop will be dependent in part upon the charges carried by previously formed drops in the jet drop stream, since these previously formed drops are still relatively close to the fluid filament tip. This effect, known as drop-to-drop "cross talk", produces inaccurate charging of drops, causing inaccurate deflection of the drops in the deflection field.
A number of approaches have been taken to eliminate or compensate for such drop charging and placement problems. In U.S. Pat. No. 3,588,906, issued June 28, 1971, to Van Brimer et al, an ink jet printer is disclosed in which a tachometer arrangement provides pulses at a frequency corresponding to the rate of movement of the print receiving medium past the print station. These pulses are utilized to control the timing of the application of charging signals to the charge electrode, as well as to control the frequency of the piezoelectric transducer which provides drop stimulation. While providing precise control of drop charging and stimulation in correspondence with movement of the print receiving medium, such an arrangement results in the application to the piezlelectric stimulator of a driving signal which fluctuates in frequency in dependence upon the fluctuation in the speed of the print receiving medium.
It should be appreciated, however, that the amplitude response of the piezoelectric transducer is not independent of the stimulation frequency. As a consequence, a fluctuation in stimulation amplitude and a corresponding fluctuation in fluid filament length may result from varying the transducer driving frequency during operation of the printer. If the fluid filament changes in length, the point of drop formation from the fluid filament tip is altered, thus changing the distance between the charge electrode and the drops being formed and, consequently, the charging efficiency of the electrode. Further, the difficulties encountered with drop-to-drop cross talk are not compensated by the Van Brimer et al system.
In U.S. Pat. No. 4,012,745, issued Mar. 15, 1977, to Brown et al, an ink jet printer is disclosed in which the piezoelectric stimulator is driven at a constant frequency, while the timing of the application of successive charging potentials to the charge electrode is controlled by a phase correction system which monitors the timing of drop formation by means of a microphone that sits just inside the mouth of the ink droplet catcher. While synchronizing the application of charging signals to the charge electrode in dependence upon the timing of drop formation, and while providing a constant frequency driving signal to the piezoelectric transducer, the Brown et al system does not provide for compensation for drop-to-drop cross talk, nor for compensation for fluctuations in the velocity of the print receiving medium.
U.S. Pat. No. 3,596,275, issued July 27, 1971, to Sweet, teaches a variable level charging system for an ink jet signal recording system in which printing is accomplished with only every third drop of a jet drop stream. The intermediate pairs of drops, termed "guard drops", are uniformly charged such that they are deflected to the catcher. By this arrangement, drop-to-drop cross talk is substantially reduced during charging of each of the print drops since the charging effects of the preceding two guard drops will always be the same and, therefore, may be taken into account.
It is seen, therefore, that there is a need for a system in which the above noted difficulties of the prior art are reduced so as to enhance the quality of the print image produced by an ink jet printer.